1. Field of the Invention
The present invention relates to a liquid crystal display device that uses OCB (Optically Compensated Bend) liquid crystal display elements in order to display an image.
2. Description of the Related Art
The liquid crystal display device includes a liquid crystal display panel that provides a matrix array of OCB liquid crystal display elements. The liquid crystal display panel includes an array substrate in which a plurality of pixel electrodes are covered with an alignment film and arrayed in a matrix, a counter-substrate in which a counter-electrode is covered with an alignment film and disposed so as to face the pixel electrodes, and a liquid crystal layer that is held between the array substrate and the counter-substrate in contact with each of the alignment films. Further, the liquid crystal display panel is configured such that a pair of polarizers are attached to the array substrate and the counter-substrate via optical retardation plates (see Jpn. Pat. Appln. KOKAI Publication No. 9-185032, for instance). Each of the OCB liquid crystal display elements constitutes a pixel in a range of the associated pixel electrode. In the OCB liquid crystal display element, the alignment state of liquid crystal molecules needs to be transitioned from a splay alignment to a bend alignment capable of displaying an image, with the application of a transition voltage that is different from a normal driving voltage.
FIG. 35 shows an example of the structure of a conventional liquid crystal display device 90. In the liquid crystal display device 90, a power supply circuit 34, a controller 37, a source driver 38, a gate driver 39, a counter-electrode driver 40 and a transition voltage setting unit 97 are further provided in order to drive a matrix array of OCB liquid crystal display elements provided on a liquid crystal display (LCD) panel 41.
FIG. 36 illustrates the operation of the liquid crystal display device 90. If the power supply circuit 34 is turned on, the transition voltage setting unit 97 sets, during a transition period 5, a transition voltage 92 for causing the alignment state of liquid crystal molecules to be transitioned from the splay alignment to the bend alignment. The controller 37 controls the source driver 38, gate driver 39 and counter-electrode driver 40 such that the transition voltage 92 is applied to the OCB liquid crystal display elements. The transition voltage 92 is a DC voltage having a positive or negative polarity. In a display period 8 that follows the transition period 5, the controller 37 controls the source driver 38, gate driver 39 and counter-electrode driver 40 such that the OCB liquid crystal display elements display an image corresponding to a display signal that is in sync with a sync signal.
In the above-described structure, however, the transition voltage 92 is applied to the OCB liquid crystal display elements as DC voltage in the transition period 5 immediately after supply of power. If the application of the transition voltage is repeated each time the power is supplied, there arises a problem that liquid crystal molecules gradually become unable to be aligned in a state of the bend alignment completely transitioned from the splay alignment.
Further, with the use of DC voltage as the transition voltage, a reference voltage value for AC application would shift when the OCB liquid crystal display elements are AC driven in the display period 8 that follows the transition period 5. Consequently, the image display quality deteriorates due to flicker.